As we age, the pattern of chemical markings on our DNA changes. Each gene becomes more or less methylated, that is, they have methyl chemical groups added or removed. This generally increases or decreases gene expression. The whole process is known as epigenetics.

Steve Horvath at the University of California, Los Angeles, and his colleagues have used these changes to estimate a person's age. To do so, they first performed a detailed statistical analysis of methylation patterns in 7844 healthy tissue samples from 51 different types of tissue. The tissue covered a range of ages – from fetuses to people 101 years old.